Numerical modeling of the off-design performance of a transcritical CO₂ two-phase ejector cycle

Ejector assisted cycles are known to offer significant COP and capacity improvements, especially for transcritical CO₂ systems, when operated at the ejector design conditions. However, significant deviation from design conditions can lead to poor ejector and cycle performance. In this paper, a numerical model of a transcritical CO₂ ejector cycle accounting for off-design ejector performance is developed. Experimental data from a CO₂ ejector cycle, taken over a range of compressor speeds and ambient temperatures, is used to develop empirical models for the ejector and compressor. The evaporator in the system is a counterflow, microchannel heat exchanger and is simulated using a finite difference model capable of accounting for the heat transfer and pressure drop throughout the heat exchanger. The models of the individual components and cycle are validated using experimental data; good agreement between model and experiment is obtained.